The NSW government has declared the second state of emergency of the bushfire season.
Premier Gladys Berejiklian says residents need to follow the advice of authorities over the next week, with extra diversions and road closures to be installed, and revise their Christmas plans.
The last state of emergency, which also ran for seven days in mid-November amid "catastrophic" fire risk, was the first implemented in NSW since 2013.
The state of emergency allows powers to be transferred from the government to Rural Fire Service commissioner Shane Fitzsimmons, including the ability to control and co-ordinate the allocation of resources and evacuate people from property within a declared area.
Mr Fitzsimmons will also be able to direct any NSW government agency to conduct or refrain from conducting its functions.
"The decision to declare a state of emergency is not taken lightly. It normally only goes for seven days and you have to make sure conditions are serious enough for that declaration to be made," Ms Berejiklian told reporters on Thursday morning.
"You can't afford for people to be complacent. You only declare states of emergency when it's absolutely necessary and on expert advice."
Dangerous fire conditions are threatening large parts of the state as up to 100 bushfires rage and communities prepare for a day of high temperatures.
The RFS has extreme fire danger warnings in place for the Greater Sydney, Illawarra and Shoalhaven and Southern Ranges areas.
Mr Fitzsimmons said on fire behaviour would be erratic due to volatile winds.
"Whilst today is going to be a very bad day, dangerous day here in NSW, the forecast is that Saturday will be even worse, so we're talking about very dangerous conditions over the coming days," he said.
"We have sought the support of the premier in ensuring we have invoked the highest level of instrument available in the state to ensure the fullest level of co-operation and integration between the multi-agency, whole-of-government team effort."
A statewide total fire ban announced on Tuesday will remain in place until at least midnight on Saturday.
Mr Fitzsimmons said the main concern on Thursday would be fires surrounding Ulladulla and Batemans Bay on the south coast, particularly the 100,000-hectare Currowan fire, as well as blazes behind the Warragamba Dam.
The huge Gospers Mountain fire northwest of Sydney was continuing to burn out of control on multiple fronts in the Wollemi National park.
The 410,000ha fire was a watch and act level on Thursday morning but there was no immediate threat to property.
The Bureau of Meteorology expects most of inland NSW will see temperatures in the mid 40s while areas west of the Sydney CBD are expected to top 41C.
Of the fires burning around the state, more than half are uncontained.
Mr Fitzsimmons said Australian Defence Force aircraft, logistics and operations expertise would be put to use fighting fires on Thursday.
"As the premier has quite rightly indicated, we know this is a precious time of year for families to connect and unite and spend time with one another but I would really reinforce the message of the premier to re-evaluate, consider and assess your travel options," Mr Fitzsimmons said.
© AAP 2019